Figures

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Introduction

The End-Permian Mass Extinction (EPME) was the greatest biological and ecological crisis in Earth history (Shen et al., 2011

Shen, S.-Z., Crowley, J.L., Wang, Y., Bowring, S.A., Erwin, D.H., Sadler, P.M., Cao, C.-Q., Rothman, D.H., Henderson, C.M., Ramezani, J., Zhang, H., Shen, Y., Wang, X.-D., Wang, W., Mu, L., Li, W.-Z., Tang, Y.-G., Liu, X.-L., Liu, L.-J., Zeng, Y., Jiang, Y.-F., Jin, Y.-G. (2011) Calibrating the End-Permian Mass Extinction. Science 334, 1367–1372. https://doi.org/10.1126/science.1213454

). Following the EPME, the ecosystem recovery was delayed due to globally recurrent climate warming (Sun et al., 2012

Sun, Y., Joachimski, M.M., Wignall, P.B., Yan, C., Chen, Y., Jiang, H., Wang, L., Lai, X. (2012) Lethally Hot Temperatures During the Early Triassic Greenhouse. Science 338, 366–370. https://doi.org/10.1126/science.1224126

; Benton and Newell, 2014

Benton, M.J., Newell, A.J. (2014) Impacts of global warming on Permo-Triassic terrestrial ecosystems. Gondwana Research 25, 1308–1337. https://doi.org/10.1016/j.gr.2012.12.010

) and harsh marine and terrestrial environments (Chen and Benton, 2012

Chen, Z.-Q., Benton, M.J. (2012) The timing and pattern of biotic recovery following the end-Permian mass extinction. Nature Geoscience 5, 375–383. https://doi.org/10.1038/ngeo1475

). Marine and terrestrial ecosystems recovered globally ∼8–10 Myr after the EPME (Chen and Benton, 2012

Chen, Z.-Q., Benton, M.J. (2012) The timing and pattern of biotic recovery following the end-Permian mass extinction. Nature Geoscience 5, 375–383. https://doi.org/10.1038/ngeo1475

; Zhao et al., 2020

Zhao, X., Zheng, D., Xie, G., Jenkyns, H.C., Guan, C., Fang, Y., He, J., Yuan, X., Xue, N., Wang, H., Li, S., Jarzembowski, E.A., Zhang, H., Wang, B. (2020) Recovery of lacustrine ecosystems after the end-Permian mass extinction. Geology 48, 609–613. https://doi.org/10.1130/G47502.1

). Among them, the Ordos Basin in North China was a trophically multi-levelled lacustrine ecosystem and was identified as the globally earliest recovered lacustrine basin (Zhao et al., 2020

Zhao, X., Zheng, D., Xie, G., Jenkyns, H.C., Guan, C., Fang, Y., He, J., Yuan, X., Xue, N., Wang, H., Li, S., Jarzembowski, E.A., Zhang, H., Wang, B. (2020) Recovery of lacustrine ecosystems after the end-Permian mass extinction. Geology 48, 609–613. https://doi.org/10.1130/G47502.1

).

Additionally, the Ordos Basin preserves a suit of high-quality hydrocarbon source rocks with well-developed maturity and extremely high total organic carbon (TOC) content up to 40 wt. % (averaging at 1.8 wt. % for hydrocarbon source rocks), currently producing the largest amount of shale oil in China (>70 %), and which may be closely associated with the ecosystem recovery from the EPME (Liu et al., 2021

Liu, H., Qiu, Z., Zou, C., Fu, J., Zhang, W., Tao, H., Li, S., Zhou, S., Wang, L., Chen, Z.-Q. (2021) Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction. Global and Planetary Change 204, 103559. https://doi.org/10.1016/j.gloplacha.2021.103559

). The mechanism for the abnormally high accumulation of organic matter in the Middle Triassic Ordos Basin remains highly debated, including (1) elevated primary productivity providing organic matter Yuan et al., 2017

Yuan, W., Liu, G., Stebbins, A., Xu, L., Niu, X., Luo, W., Li, C. (2017) Reconstruction of redox conditions during deposition of organic-rich shales of the Upper Triassic Yanchang Formation, Ordos Basin, China. Palaeogeography, Palaeoclimatology, Palaeoecology 486, 158–170. https://doi.org/10.1016/j.palaeo.2016.12.020

; Liu et al., 2021

Liu, H., Qiu, Z., Zou, C., Fu, J., Zhang, W., Tao, H., Li, S., Zhou, S., Wang, L., Chen, Z.-Q. (2021) Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction. Global and Planetary Change 204, 103559. https://doi.org/10.1016/j.gloplacha.2021.103559

), (2) a reducing environment that is favourable for organic matter preservation (B. Zhang et al., 2021

Zhang, B., Mao, Z., Zhang, Z., Yuan, Y., Chen, X., Shi, Y., Liu, G., Shao, X. (2021) Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China. Petroleum Exploration and Development 48, 1304–1314. https://doi.org/10.1016/S1876-3804(21)60288-4

), and (3) a combination of both (Li et al., 2020

Li, S., Niu, X., Liu, G., Li, J., Sun, M., You, F., He, H. (2020) Formation and accumulation mechanism of shale oil in the 7th member of Yanchang Formation, Ordos Basin. Oil & Gas Geology 41, 719–729. https://doi.org/10.11743/ogg20200406

). These debates likely resulted from the assumptions that multiple palaeo-oceanic proxies previously used are influenced by active volcanism (Wang et al., 2021

Wang, C., Wang, Q., Chen, G., Chen, D. (2021) Influence of volcanism on the development of black shales in the Chang 7 Member of Yanchang Formation in the Ordos Basin. International Journal of Earth Sciences 110, 1939–1960. https://doi.org/10.1007/s00531-021-02050-8

) or are inappropriate for lacustrine environments (Scott et al., 2012

Scott, J.J., Buatois, L.A., Mángano, M.G. (2012) Chapter 13 - Lacustrine Environments. In: Knaust, D., Bromley, R.G. (Eds.) Trace Fossils as Indicators of Sedimentary Environments. Developments in Sedimentology 64, First Edition, Elsevier, Amsterdam, 379–417. https://doi.org/10.1016/B978-0-444-53813-0.00013-7

).

Cadmium (Cd) isotope system is an effective tool for tracing primary productivity in modern and ancient oceans (Sweere et al., 2020

Sweere, T.C., Dickson, A.J., Jenkyns, H.C., Porcelli, D., Ruhl, M., Murphy, M.J., Idiz, E., van den Boorn, S.H.J.M., Eldrett, J.S., Henderson, G.M. (2020) Controls on the Cd-isotope composition of Upper Cretaceous (Cenomanian–Turonian) organic-rich mudrocks from south Texas (Eagle Ford Group). Geochimica et Cosmochimica Acta 287, 251–262. https://doi.org/10.1016/j.gca.2020.02.019

; Chen et al., 2021

Chen, L., Little, S.H., Kreissig, K., Severmann, S., McManus, J. (2021) Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones. Frontiers in Earth Science 9, 623720. https://doi.org/10.3389/feart.2021.623720

), as the biogeochemical cycle of Cd and associated isotope fractionations are mainly controlled by biological pumps (Fig. S-1; e.g., Guinoiseau et al., 2019

Guinoiseau, D., Galer, S.J.G., Abouchami, W., Frank, M., Achterberg, E.P., Haug, G.H. (2019) Importance of Cadmium Sulfides for Biogeochemical Cycling of Cd and Its Isotopes in Oxygen Deficient Zones—A Case Study of the Angola Basin. Global Biogeochemical Cycles 33, 1746–1763. https://doi.org/10.1029/2019GB006323

; Xie et al., 2019

Xie, R.C., Rehkämper, M., Grasse, P., van de Flierdt, T., Frank, M., Xue, Z. (2019) Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific. Earth and Planetary Science Letters 512, 134–146. https://doi.org/10.1016/j.epsl.2019.02.001

; see Supplementary Information). In this study, we report Cd isotope and redox sensitive trace element (Mo and U) data for bulk black shales of the Middle Triassic successions from two wells in the Ordos Basin, in order to explore (1) the dominant mechanism for the abnormally high accumulation of organic matter in the Middle Triassic Ordos Basin, and (2) its contribution to the ecosystem recovery from the EPME.

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Samples and Analytical Methods

In the Ordos Basin, the Triassic Yanchang Formation can be divided into 10 members, Chang 1 to Chang 10 from top to bottom. Among them, the Chang 7 Member is further subdivided into three sub-members, Chang 7₁, Chang 7₂, and Chang 7₃ from top to bottom. The Chang 7₃ sub-member is dominated by black shales acting as high-quality hydrocarbon source rocks. Black shale samples were collected from the Chang 7₃ sub-member at the Ning 70 (N70) and Feng 75 (F75) wells, which are located in the southern and northern basin, respectively (Fig. 1). The samples were subjected to analyses of thermal maturities, TOC contents, major and trace element contents, and Cd isotope compositions (δ¹¹⁴Cd). Detailed description of geological background and samples, as well as analytical procedures, can be seen in Supplementary Information.

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Effects of Sedimentation Rate, Detrital Contamination, Volcanic Activity, and Thermal Maturation on Geochemical Indicators

The measured TOC, Al, Cd, Mo, and U contents and δ¹¹⁴Cd values are listed in Table S-1. Sedimentation rate, detrital contamination, volcanic activity, and thermal maturation may influence these geochemical indicators, necessitating their evaluation.

Sedimentation rate. Sedimentation rate can control organic matter accumulation in sediments, which further influences the biogeochemical cycle of metal elements, although its effect is complex (K. Zhang et al., 2021

Zhang, K., Liu, R., Liu, Z. (2021) Sedimentary sequence evolution and organic matter accumulation characteristics of the Chang 8–Chang 7 members in the Upper Triassic Yanchang Formation, southwest Ordos Basin, central China. Journal of Petroleum Science and Engineering 196, 107751. https://doi.org/10.1016/j.petrol.2020.107751

). The (La/Yb)_N ratio (normalised to the upper continental crust; UCC) is a useful proxy for sedimentation rate: higher rates when (La/Yb)_N is approaching 1, not vice versa (Arthur and Sageman, 1994

Arthur, M.A., Sageman, B.B. (1994) Marine black shales: Depositional mechanisms and environments of ancient deposits. Annual Review of Earth and Planetary Sciences 22, 499–551. https://doi.org/10.1146/annurev.ea.22.050194.002435

). The (La/Yb)_N ratios of the samples (N70, 0.7–1.3; F75, 1.2–1.5; Table S-1) exhibit no clear relationship with the TOC contents (Fig. S-2), suggesting a minimal effect of sedimentation rate.

Detrital contamination. Since we performed a complete digestion for the samples, the measured Mo, U, and Cd contents and δ¹¹⁴Cd values are mixtures of authigenic and detrital signals. As shown in Figure S-3, there is no positive correlation between Al and Mo, U or Cd contents and no clear relationship between Al and δ¹¹⁴Cd, suggesting insignificant detrital inputs. Therefore, we use following equations to obtain enrichment factors of Cd, Mo, and U, and fraction (f_auth), content (Cd_auth), and isotope composition (δ¹¹⁴Cd_auth) of authigenic Cd:

where X_EF represents enrichment factors of Cd, Mo and U, and the Cd, Mo, U and Al contents of the UCC that are 0.098 ppm, 1.5 ppm, 2.8 ppm and 8.04 wt. %, respectively (McLennan, 2001

McLennan, S.M. (2001) Relationships between the trace element composition of sedimentary rocks and upper continental crust. Geochemistry, Geophysics, Geosystems 2, 2000GC000109. https://doi.org/10.1029/2000GC000109

); the Cd/Al and δ¹¹⁴Cd values of detrital components are supposed to be similar to the UCC values, 0.0122 ppm/wt. % and 0.03 ‰ (Pickard et al., 2022

Pickard, H., Palk, E., Schönbächler, M., Moore, R.E.T., Coles, B.J., Kreissig, K., Nilsson-Kerr, K., Hammond, S.J., Takazawa, E., Hémond, C., Tropper, P., Barfod, D.N., Rehkämper, M. (2022) The cadmium and zinc isotope compositions of the silicate Earth – Implications for terrestrial volatile accretion. Geochimica et Cosmochimica Acta 338, 165–180. https://doi.org/10.1016/j.gca.2022.09.041

), respectively. The calculated results are listed in Table S-1, and the Cd_EF, δ¹¹⁴Cd_auth, Mo_EF, and U_EF profiles are plotted against burial depth together with the TOC profile in Figure 2.

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The N70 and F75 samples show distinct geochemical characteristics. The TOC contents of the N70 samples (7.6–32.6 wt. %) are much higher than the F75 ones (3.4–10.2 wt. %). Similarly, the N70 samples contain more authigenic Cd, Mo and U relative to the F75 ones (Cd_EF, 4.4–28.1 versus 2.7–5.2; Mo_EF, 39.9–375.2 versus 8.7–16.0; U_EF, 12.6–96.6 versus 1.5–5.4). The N70 samples record higher δ¹¹⁴Cd_auth values from 0.22 ‰ to 0.69 ‰, averaging 0.37 ± 0.26 ‰ (2 s.d., n = 14), whereas the F75 samples record lower δ¹¹⁴Cd_auth values from −0.27 ‰ to +0.28 ‰ averaging at 0.06 ± 0.29 ‰ (2 s.d., n = 20).

Volcanic activity. Volcanic ash layers ubiquitously occur in the lower Chang 7₃ sub-member, whose thicknesses gradually decrease from southwest to northeast, indicating that volcanic activities mainly occurred on/to the southwestern edge of the basin (Wang et al., 2021

Wang, C., Wang, Q., Chen, G., Chen, D. (2021) Influence of volcanism on the development of black shales in the Chang 7 Member of Yanchang Formation in the Ordos Basin. International Journal of Earth Sciences 110, 1939–1960. https://doi.org/10.1007/s00531-021-02050-8

). Indeed, the N70 samples contain more tuff layers than the F75 samples (B. Zhang et al., 2021

Zhang, B., Mao, Z., Zhang, Z., Yuan, Y., Chen, X., Shi, Y., Liu, G., Shao, X. (2021) Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China. Petroleum Exploration and Development 48, 1304–1314. https://doi.org/10.1016/S1876-3804(21)60288-4

; Lin et al., 2023

Lin, M., Xi, K., Cao, Y., Liu, K., Zhu, R. (2023) Periodic paleo-environment oscillation on multi-timescales in the Triassic and their significant implications for algal blooms: A case study on the lacustrine shales in Ordos Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 612, 111376. https://doi.org/10.1016/j.palaeo.2022.111376

). More volcanic materials in the N70 samples might result in greater enrichments of Cd, Mo and U relative to the F75 ones (Fig. 2), hampering palaeo-environmental reconstruction by these elemental and isotopic indicators. However, we argue against this possibility because the Mo (0.039–0.043 ppm), U (4.8–5.7 ppm), and Cd (0.118–0.134 ppm) contents of the volcanic materials in the Chang 7₃ sub-member are much lower than those of our samples, especially for the N70 ones (Tian, 2023

Tian, K.W. (2023) Discussion on the spatial and temporal distribution, provenance characteristics of tuff in Ordos Basin and the relationship between biological hysteresis recoveries in early Triassic. Master’s Thesis, Taiyuan University of Technology. https://doi.org/10.27352/d.cnki.gylgu.2023.000605

). Therefore, we conclude that the direct impact of volcanic materials in the N70 and F75 samples is not responsible for their geochemical differences.

Thermal maturation. The studied samples have well-developed thermal maturities, as indicated by moderate Rock-Eval T_max (temperature of maximum hydrocarbon yield; N70, 433–437 °C; F75, 434–446 °C). The thermal maturation may lead to loss of elements associated with organic matter and may alter primary isotope compositions. However, the pyrolysis experiments conducted by Dickson et al. (2020)

Dickson, A.J., Idiz, E., Porcelli, D., van den Boorn, S.H.J.M. (2020) The influence of thermal maturity on the stable isotope compositions and concentrations of molybdenum, zinc and cadmium in organic-rich marine mudrocks. Geochimica et Cosmochimica Acta 287, 205–220. https://doi.org/10.1016/j.gca.2019.11.001

have demonstrated that the effect of thermal maturation is minimal on the contents and isotope compositions of Mo and Cd in organic-rich mudrocks. This effect on the Mo_EF, U_EF, Cd_EF and δ¹¹⁴Cd_auth variabilities of the samples can be further eliminated by the absence of a clear relationship between the T_max versus the above geochemical indicators (Fig. S-4).

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Palaeo-redox and -productivity Reconstructions

The accumulation of organic matter in sediments is controlled by the competition between the production and consumption of organic matter: high primary productivity can provide the basis for accumulation of organic matter, whereas reducing environments can diminish the consumption of organic matter and promote organic matter preservation in sediments (e.g., Demaison and Moore, 1980

Demaison, G.J., Moore, G.T. (1980) Anoxic environments and oil source bed genesis. Organic Geochemistry 2, 9–31. https://doi.org/10.1016/0146-6380(80)90017-0

; Pedersen and Calvent, 1990

Pedersen, T.F., Calvert, S.E. (1990) Anoxia vs. Productivity: What controls the formation of organic-carbon-rich sediments and sedimentary rocks? AAPG Bulletin 74, 454–466. https://doi.org/10.1306/0c9b232b-1710-11d7-8645000102c1865d

; Bohacs et al., 2005

Bohacs, K.M., Grabowski Jr., G.J., Carroll, A.R., Mankiewicz, P.J., Miskell-Gerhardt, K.J., Schwalbach, J.R., Wegner, M.B., Simo, J.A. (2005) Production, Destruction, and Dilution—The Many Paths to Source-Rock Development. In: Harris, N.B. (Ed.) The Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and Consequences. Society for Sedimentary Geology, Tulsa, Special Publication 82, 61–101. https://doi.org/10.2110/pec.05.82.0061

). Therefore, it is imperative to reconstruct the primary productivity and redox condition of the Middle Triassic Ordos Basin, to comprehend the dominant mechanism for the abnormally high accumulation of organic matter.

Redox conditions. Authigenic enrichment factors of redox sensitive trace elements (Mo and U) in sediments are useful palaeo-redox proxies: oxic or suboxic sediments generally exhibit little authigenic Mo and U enrichments, whereas anoxic or euxinic environments exhibit strong authigenic Mo-U enrichments due to removal by organic matter and/or H₂S (Algeo and Tribovillard, 2009

Algeo, T.J., Tribovillard, N. (2009) Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation. Chemical Geology 268, 211–225. https://doi.org/10.1016/j.chemgeo.2009.09.001

). Therefore, the lower Mo_EF and U_EF values of the F75 samples indicate a suboxic or anoxic condition with a deeper chemocline; in contrast, the much higher Mo_EF and U_EF values of the N70 samples suggest an anoxic or euxinic environment with a shallower chemocline (Fig. 3a). Although pyrite crystals have been found in the N70 samples (Lin et al., 2023

Lin, M., Xi, K., Cao, Y., Liu, K., Zhu, R. (2023) Periodic paleo-environment oscillation on multi-timescales in the Triassic and their significant implications for algal blooms: A case study on the lacustrine shales in Ordos Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 612, 111376. https://doi.org/10.1016/j.palaeo.2022.111376

), the positive Mo_EF-TOC and U_EF-TOC correlations (Fig. S-5a,b) reveal that the Mo and U in the southern Ordos Basin were mainly removed by organic matter rather than by H₂S. It further indicates that the southern Ordos Basin was dominated by anoxia possibly with episodic euxinia.

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The Mo_EF-U_EF co-variation pattern, together with the comparison of the Mo_EF/U_EF ratio with the average weight Mo/U ratio of modern seawater, is widely used to discriminate marine redox environments (Algeo and Tribovillard, 2009

Algeo, T.J., Tribovillard, N. (2009) Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation. Chemical Geology 268, 211–225. https://doi.org/10.1016/j.chemgeo.2009.09.001

). Considering that the Ordos Basin is a lacustrine basin, we used the average weight Mo/U ratio of modern river water (Mo_RW/U_RW = 1.13; Gaillardet et al., 2003

Gaillardet, J., Viers, J., Dupré, B. (2003) Trace Elements in River Waters. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. First Edition, Elsevier, Amsterdam, 5, 225–272. https://doi.org/10.1016/B0-08-043751-6/05165-3

) to replace modern seawater value. Most data fit the coefficient line of the triple Mo_RW/U_RW ratio (Fig. S-6), suggesting that the riverine input should be the dominant source for authigenic Mo, U, and likely Cd of the N70 and F75 samples. Moreover, most Mo_EF/U_EF ratios of the F75 samples are obviously higher than the triple Mo_RW/U_RW ratio, possibly caused by the control of Fe-Mn (oxyhydr)oxides due to the stronger adsorption capacity of dissolved Mo than that of U (Algeo and Tribovillard, 2009

Algeo, T.J., Tribovillard, N. (2009) Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation. Chemical Geology 268, 211–225. https://doi.org/10.1016/j.chemgeo.2009.09.001

). This further demonstrates a deeper chemocline near the sediment-water interface in the northern basin (Fig. 3a).

Primary productivity. In oxygen deficient environments, partial remineralisation of organic matter would release Cd that subsequently precipitates as CdS in reducing microenvironments within sinking organic matter (Guinoiseau et al., 2019

Guinoiseau, D., Galer, S.J.G., Abouchami, W., Frank, M., Achterberg, E.P., Haug, G.H. (2019) Importance of Cadmium Sulfides for Biogeochemical Cycling of Cd and Its Isotopes in Oxygen Deficient Zones—A Case Study of the Angola Basin. Global Biogeochemical Cycles 33, 1746–1763. https://doi.org/10.1029/2019GB006323

), in an anoxic sediment-water interface (Xie et al., 2019

Xie, R.C., Rehkämper, M., Grasse, P., van de Flierdt, T., Frank, M., Xue, Z. (2019) Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific. Earth and Planetary Science Letters 512, 134–146. https://doi.org/10.1016/j.epsl.2019.02.001

), or in an anoxic sediment (Chen et al., 2021

Chen, L., Little, S.H., Kreissig, K., Severmann, S., McManus, J. (2021) Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones. Frontiers in Earth Science 9, 623720. https://doi.org/10.3389/feart.2021.623720

). These processes could supply an additional Cd source to sediments (beyond that supplied by organic matter) and modify the primary δ¹¹⁴Cd signal inheriting from surface water (Chen et al., 2021

Chen, L., Little, S.H., Kreissig, K., Severmann, S., McManus, J. (2021) Isotopically Light Cd in Sediments Underlying Oxygen Deficient Zones. Frontiers in Earth Science 9, 623720. https://doi.org/10.3389/feart.2021.623720

). However, the Cd_auth/TOC ratios of the N70 (0.022–0.152 ppm/wt. %) and F75 (0.027–0.112 ppm/wt. %) samples are mostly lower than the values of exported organic particles in non-HNLC (high nutrient low chlorophyll) regions (0.075–0.75 ppm/wt. %; Fig. S-7). This indicates that the Cd in the samples was dominantly associated with organic matter (Sweere et al., 2020

Sweere, T.C., Dickson, A.J., Jenkyns, H.C., Porcelli, D., Ruhl, M., Murphy, M.J., Idiz, E., van den Boorn, S.H.J.M., Eldrett, J.S., Henderson, G.M. (2020) Controls on the Cd-isotope composition of Upper Cretaceous (Cenomanian–Turonian) organic-rich mudrocks from south Texas (Eagle Ford Group). Geochimica et Cosmochimica Acta 287, 251–262. https://doi.org/10.1016/j.gca.2020.02.019

), which is also supported by the positive Cd_EF-TOC correlation (Fig. S-5c). In other words, CdS precipitation induced by the remineralisation of organic matter could be negligible. Therefore, our Cd isotope data can reflect primary productivity in the Middle Triassic Ordos Basin.

As shown in Figure 4a,b, the δ¹¹⁴Cd_auth values of the F75 samples are generally similar to those of black shales reported hitherto in literature; in contrast, the δ¹¹⁴Cd_auth values of the N70 samples are obviously higher, and are consistent with the estimated value of riverine input (0.36 ± 0.10 ‰; see Supplementary Information), which was likely the dominant Cd source of the Ordos Basin during the Middle Triassic. Given that the Cd accumulation in the Ordos Basin was mainly controlled by organic matter accumulation, the Cd_auth and δ¹¹⁴Cd_auth variabilities in an individual well and their differences between the two studied wells should be determined by varying degrees of biological Cd utilisation following a Rayleigh-type distillation:

where δ¹¹⁴Cd_AP is the δ¹¹⁴Cd of accumulated product (δ¹¹⁴Cd_auth); δ¹¹⁴Cd_reactant,0 is the initial δ¹¹⁴Cd value that is set to the estimated average δ¹¹⁴Cd value of the riverine input (0.36 ‰); f_remain denotes the proportion of remaining Cd; and Δ_P–R is the Cd isotope fractionation during Cd removal by primary producers, which is estimated from −0.6 ‰ to −0.2 ‰ (see Supplementary Information) and thus tentatively set to −0.6 ‰, −0.4 ‰ and −0.2 ‰. We can drive δ¹¹⁴Cd_AP as a function of f_remain (Fig. 4c). The extremely high δ¹¹⁴Cd_auth values of the N70 samples reflect near-quantitative removal of Cd by primary producers in the southern basin, whereas the relatively low δ¹¹⁴Cd_auth values of the F75 samples reflect non-quantitative (∼53–76 %) Cd removal in the North (the data cannot be modelled when Δ_P–R = −0.2 ‰). Therefore, during the Middle Triassic, the primary productivity in the southern Ordos Basin was abnormally high.

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Abnormally High Accumulation of Organic Matter and Ecosystem Recovery from the EPME

Based on the investigation above, we propose a model for the abnormally high accumulation of organic matter in the Middle Triassic Ordos Basin (Fig. 3). The Qinling orogenesis led to frequent volcanic activity on/to the southwestern edge of the Ordos Basin (e.g., Wang et al., 2021

Wang, C., Wang, Q., Chen, G., Chen, D. (2021) Influence of volcanism on the development of black shales in the Chang 7 Member of Yanchang Formation in the Ordos Basin. International Journal of Earth Sciences 110, 1939–1960. https://doi.org/10.1007/s00531-021-02050-8

). The regional uplift of the continental crust and releasing of massive amounts of CO₂ (which enhanced chemical weathering under a warm and humid climate) would have elevated nutrient supply into the southern basin via riverine transport, which may have decreased laterally from south to north (B. Zhang et al., 2021

Zhang, B., Mao, Z., Zhang, Z., Yuan, Y., Chen, X., Shi, Y., Liu, G., Shao, X. (2021) Black shale formation environment and its control on shale oil enrichment in Triassic Chang 7 Member, Ordos Basin, NW China. Petroleum Exploration and Development 48, 1304–1314. https://doi.org/10.1016/S1876-3804(21)60288-4

). The northern basin was also supplied by rivers from the north, which were less influenced by volcanic activities. In the southern basin where nutrients were sufficient, abnormally elevated primary productivity near-quantitatively removed dissolved Cd. The remineralisation of organic matter in deeper water consumed significant amounts of O₂, resulting in local anoxia with episodic euxinia (shallower chemocline) and further promoting organic matter preservation in sediments (Fig. 3b). In the northern basin where the nutrient supply was relatively low, moderate primary productivity non-quantitatively removed dissolved Cd and lead to less consumption of O₂, causing suboxic or anoxic conditions with a deeper chemocline near the water-sediment interface (Fig. 3c).

The environment of the northern basin is unfavourable for organic matter preservation (Demaison and Moore, 1980

Demaison, G.J., Moore, G.T. (1980) Anoxic environments and oil source bed genesis. Organic Geochemistry 2, 9–31. https://doi.org/10.1016/0146-6380(80)90017-0

). However, the TOC contents of the F75 samples (3.4–10.2 wt. %) are greatly higher than those of other hydrocarbon source rocks in both lacustrine and marine systems (average 1.8 wt. % TOC). This indicates that primary productivity played a first order role in the accumulation of organic matter in the northern Ordos Basin. In addition, the primary productivity in the southern Ordos Basin was much greater, simultaneously resulting in local anoxia and episodic euxinia likely caused by the remineralisation of organic matter. Both factors ultimately led to the abnormally high TOC contents in the N70 samples. Therefore, we conclude that abnormally high primary productivity was the dominant mechanism for the accumulation of organic matter in the Middle Triassic Ordos Basin, which could further be promoted by local anoxia. This case is completely different from the Green River Formation in the Uinta Basin (Utah), an Eocene stratum also with extremely high TOC contents (up to 34 wt. %), which was attributed to density stratification and anoxia in a saline lake system (Birgenheier et al., 2020

Birgenheier, L.P., Vanden Berg, M.D., Plink-Björklund, P., Gall, R.D., Rosencrans, E., Rosenberg, M.J., Toms, L.C., Morris, J. (2020) Climate impact on fluvial-lake system evolution, Eocene Green River Formation, Uinta Basin, Utah, USA. GSA Bulletin 132, 562–587. https://doi.org/10.1130/B31808.1

).

Massive burial of organic matter dominantly induced by elevated primary productivity have been demonstrated to be widespread in the Middle Triassic Ordos Basin (this study; e.g., Yuan et al., 2017

Yuan, W., Liu, G., Stebbins, A., Xu, L., Niu, X., Luo, W., Li, C. (2017) Reconstruction of redox conditions during deposition of organic-rich shales of the Upper Triassic Yanchang Formation, Ordos Basin, China. Palaeogeography, Palaeoclimatology, Palaeoecology 486, 158–170. https://doi.org/10.1016/j.palaeo.2016.12.020

; Liu et al., 2021

Liu, H., Qiu, Z., Zou, C., Fu, J., Zhang, W., Tao, H., Li, S., Zhou, S., Wang, L., Chen, Z.-Q. (2021) Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction. Global and Planetary Change 204, 103559. https://doi.org/10.1016/j.gloplacha.2021.103559

). The elevated primary productivity could have laid the foundation for a complex food chain and structured a trophically multi-levelled ecosystem in the Middle Triassic Ordos Basin (Chen and Benton, 2012

Chen, Z.-Q., Benton, M.J. (2012) The timing and pattern of biotic recovery following the end-Permian mass extinction. Nature Geoscience 5, 375–383. https://doi.org/10.1038/ngeo1475

; Zhao et al., 2020

Zhao, X., Zheng, D., Xie, G., Jenkyns, H.C., Guan, C., Fang, Y., He, J., Yuan, X., Xue, N., Wang, H., Li, S., Jarzembowski, E.A., Zhang, H., Wang, B. (2020) Recovery of lacustrine ecosystems after the end-Permian mass extinction. Geology 48, 609–613. https://doi.org/10.1130/G47502.1

). In contrast, massive burial of organic matter could have emerged as a substantial carbon sink to accelerate global climate cooling (Xu et al., 2017

Xu, W., Ruhl, M., Jenkyns, H.C., Hesselbo, S.P., Riding, J.B., Selby, D., Naafs, B.D.A., Weijers, J.W.H., Pancost, R.D., Tegelaar, E.W., Idiz, E.F. (2017) Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event. Nature Geoscience 10, 129–134. https://doi.org/10.1038/ngeo2871

; Liu et al., 2021

Liu, H., Qiu, Z., Zou, C., Fu, J., Zhang, W., Tao, H., Li, S., Zhou, S., Wang, L., Chen, Z.-Q. (2021) Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction. Global and Planetary Change 204, 103559. https://doi.org/10.1016/j.gloplacha.2021.103559

). Both factors collectively should make a large contribution to the globally earliest ecosystem recovery of the Ordos Basin from the EPME.

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Conclusions

We reconstruct redox conditions and primary productivity in the Middle Triassic Ordos Basin, which were laterally heterogeneous. The northern basin was suboxic or anoxic with a deeper chemocline near water-sediment interface but preserved considerable organic matter due to high primary productivity. In the southern basin, much higher primary productivity likely induced by active volcanism may have led to abnormally high accumulation of organic matter, which was further promoted by local anoxia and episodic euxinia. We thus propose that rather than the reducing environment, the abnormally elevated primary productivity should play a first order role in the accumulation of organic matter in the Middle Triassic Ordos Basin. In addition, the elevated primary productivity and massive burial of organic matter in the Ordos Basin were likely to make a large contribution to the ecosystem recovery from the EPME.

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Acknowledgements

This study is supported by the National Key Research and Development Program of China (2021YFA0718200), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB41000000), the National Natural Science Foundation of China Program (42472144), and the Science and Technology Research Project of the China National Petroleum Corporation (2024DJ87). We are grateful to the editor, Claudine Stirling, the reviewer, Simon Hohl, and the other anonymous reviewer for their constructive comments that greatly improved this manuscript.

Editor: Claudine Stirling

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References

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Supplementary Information

The Supplementary Information includes:

• Cd Isotope System as a Palaeo-productivity Proxy
• Geological Background and Samples
• Analytical Methods
• Table S-1
• Figures S-1 to S-7
• Supplementary Information References

Download the Supplementary Information (PDF)

Download Table S-1 (.xlsx)